As diamond is the hardest substance among those present on earth, a sintered diamond object is used in a cutting tool or a wear resistant tool. For example, Japanese National Patent Publication No. 39-020483 (Patent Document 1) and Japanese National Patent Publication No. 52-012126 (Patent Document 2) disclose a sintered diamond object obtained by sintering diamond particles with a binder composed of an iron-group metal such as Co (cobalt). In the sintered diamond object, chipping due to cleavage which is a disadvantage of monocrystalline diamond is less likely. Therefore, the sintered diamond object is widely used as a raw material for a cutting tool or the like for cutting and working a non-ferrous metal material such as an Al (aluminum)-Si (silicon) alloy.
The sintered diamond object containing diamond particle having an average particle size of not smaller than 5 μm and not larger than 100 μm attains excellent wear resistance. Meanwhile, the sintered diamond object containing fine diamond particle having an average particle size of less than 5 μm attains excellent chipping resistance. As in the case of common sintered ceramics, as the sintered diamond object contains the diamond particles having a finer and uniform particle size in a higher content (high density) and the particles are more firmly bound to each other, the diamond particle being hard particle constituting the sintered diamond object, the sintered diamond object tends to attain more excellent chipping resistance.
As a method of firmly binding the diamond particles, Patent Document 1 mentioned above discloses a method of using a binder implemented by a solvent, the solvent represented by an iron-group metal such as Co, Fe (iron) or Ni (nickel) attaining a catalyst capability for dissolving diamond powder and causing the same to re-precipitate so as to form direct bond called neck growth between the diamond powders. In addition, Japanese National Patent Publication No. 58-032224 (Japanese Patent Laying-Open No. 55-047363) (Patent Document 3) discloses a method of binding the diamond particles by means of a binder composed of carbide of a 4a-, 5a- or 6a-group metal in the periodic table.
In the sintered diamond object manufactured with the former method of generating neck growth between the diamond particles using Co or WC (tungsten carbide)-Co alloy as the binder, unlike the sintered diamond object manufactured with the latter method, the diamond particles can maintain a firm structure, even after the binder being poorer in hardness or corrosion resistance than the diamond particle is selectively worn due to mechanical wear such as rubbing wear or chemical wear such as corrosion. Therefore, the sintered diamond object manufactured with the former method is excellent in chipping resistance and wear resistance.
The binder itself composed of Co or the WC-Co alloy in the former case has hardness lower than the ceramics type binder used in the latter method, to say nothing of comparison with the diamond particle. Namely, this binder is disadvantageous in its susceptibility to wear due to mechanical rubbing.
As such, in the sintered diamond object obtained by firmly sintering ultra-fine diamond particles having an average particle size of not larger than 1 μm by using the Co alloy as the binder while maintaining a uniform texture, if the content of the diamond particles can be increased such that the content of the binder composed of Co or WC-Co alloy can be minimized, an ideal sintered diamond object attaining extremely excellent chipping resistance as well as wear resistance can be obtained.
If sintering is performed by using ultra-fine diamond particle having the particle size of not larger than 1 μm and the iron-group metal such as Co or WC (tungsten carbide)-Co as starting materials, however, abnormal growth of the diamond particle tends to frequently occur, unless a temperature and pressure condition at the time of sintering is strictly controlled. This is because the ultra-fine diamond particle is highly reactive. In addition, if a high temperature condition, which is important in promoting neck growth, is set and if diamond particle having the particle size of not larger than 2 μm is used as the starting material, abnormal particle growth is inevitable, and the sintered object including an abnormally grown particle portion cannot be cut with EDM (Electrical Discharge Machining). Moreover, mechanical strength of the diamond is also lowered due to generation of defects. Therefore, it is difficult to achieve high yield of the sintered diamond object having a particle size of not larger than 1 μm and a uniform texture.
As a method of suppressing abnormal growth of the diamond particle, a method of controlling abnormal particle growth by arranging hard particle such as WC, cBN (cubic boron nitride), SiC (silicon carbide) having hardness as high as the diamond at a grain boundary of the diamond particle is known. Such a method is disclosed, for example, in Japanese National Patent Publication No. 61-058432 (Patent Document 4), Japanese National Patent Publication No. 06-006769 (Japanese Patent Laying-Open No. 64-017836) (Patent Document 5), and Japanese Patent Laying-Open No. 2003-095743 (Patent Document 6).
Patent Document 1: Japanese National Patent Publication No. 39-020483
Patent Document 2: Japanese National Patent Publication No. 52-012126
Patent Document 3: Japanese National Patent Publication No. 58-032224 (Japanese Patent Laying-Open No. 55-047363)
Patent Document 4: Japanese National Patent Publication No. 61-058432
Patent Document 5: Japanese National Patent Publication No. 06-006769 (Japanese Patent Laying-Open No. 64-017836)
Patent Document 6: Japanese Patent Laying-Open No. 2003-095743